At present, the third generation mobile communication technique has been fully developed and is gradually put to commercial use. Compared with previous second generation mobile communication technique, it has many technical advantages and has a broad application prospect. In order to keep the third generation mobile communication technique be updated continuously and to maintain its competitiveness for a long time, so as to prolong the commercial life-span of the third generation mobile communication technique, 3GPP is currently working on a brand-new Long Term Evolution (LTE) system to meet future requirements of mobile communications.
In current LTE systems, the following two kinds of downlink reference signals are supported.
1) Cell-specific Reference Signals (CsRS), are reference signals which can be received and utilized by all users in a cell. In an LTE system, not more than 4 antenna ports, i.e., antenna ports 0, 1, 2, 3, are defined for transmitting the CsRS.
2) UE-specific Reference Signals (UsRS), are reference signals allocated to a specific user in the cell and can only be received and utilized by the specific user. In an LTE system, one antenna port, i.e., antenna port 5, is defined for transmitting the UsRS.
The antenna ports are not concrete physical antenna ports, but are virtual antenna ports mapped from concrete physical antenna ports. Meanwhile, antenna ports in the LTE system may also be used for defining space dimensions of wireless resources. The number of space dimensions equals the number of antenna ports used.
In an LTE system, a general processing procedure defined for downlink physical channels is shown in FIG. 1, including the following steps.
Step 101, scrambling. Scrambling of coded bits in each code word to be transmitted on a physical channel is performed.
Step 102, modulation. Scrambled bits are modulated to generate complex-valued modulation symbols.
Step 103, layer mapping. The complex-valued modulation symbols are mapped onto one or several transmission layers.
Step 104, precoding. The complex-valued modulation symbols on each transmission layer are precoded for transmission on one or more antenna ports.
Step 105, resource element mapping. The complex-valued modulation symbols which are precoded are mapped onto resource elements of respective antenna ports.
Step 106, generation of orthogonal frequency division modulation (OFDM) signals. For each antenna port, complex-valued time-domain OFDM signals on the antenna port are generated from all complex-valued modulation symbols mapped onto resource elements of the antenna port.
The above step 105 may further include: mapping the CsRS onto resource elements of antenna ports corresponding to the CsRS to form complex-valued modulation symbols corresponding to the CsRS.
After being generated, the complex-valued time-domain OFDM signals may be transmitted through the downlink physical channels.
In existing 3GPP specifications, beamforming has been accepted as one of downlink transmission schemes. But the existing specifications do not give specific solution for beamforming transmission. And the above processing procedure does not provide sufficient support for beamforming transmission.
The reason is that the concept of beamforming has not been specified in the existing processing procedure. And there is also no detailed implementation method in the existing processing procedure. For multi-user data transmission, since different users may have different beamforming weights and different users are differentiated by frequencies, beamforming operations need to be performed on different frequency-domain resources. The existing technique only allows processing data of only one user each time. The implementation is complex and inefficient, and can not meet the requirements of downlink data transmission in a flexible way.